Resonance in radio frequency sheath admittance and enhanced impurity emission near the ion cyclotron frequency

Mikhail Rezazadeh, James R. Myra, Davide Curreli

Research output: Contribution to journalArticlepeer-review


Ion cyclotron resonance heating (ICRH) is of considerable interest among all auxiliary heating techniques, because it transfers power directly to ions, targets the high-density core, and involves the cheapest radio-frequency (RF) components. During ICRH operation, RF sheaths form on the ICRH antenna itself, nearby hardware, and far-field surfaces. These sheaths are associated with large hot-spot formation and impurity emissions. This work presents high-resolution numerical modelling of RF sheaths in nuclear fusion scenarios using hPIC2, a Debye-scale particle-in-cell code. The modelling reveals a new RF sheath phenomenon which occurs when the RF is resonant with a species ion cyclotron frequency or harmonic in an oblique magnetic field. This resonance of the RF sheath causes modifications of the energy-angle distributions of the ions impacting on the walls, with consequent increase in wall impurity emission. Due to the 1 / R scaling of the magnetic field in a tokamak, such cases are possible on divertor or vacuum chamber surfaces, depending on the geometry of the tokamak and the RF heating scenario. A simple physics interpretation using a driven damped harmonic oscillator is proposed, where the ion sheath transit time plays the role of damping the RF sheath admittance. Critically, the resonance leads to increased ion heat flux at the wall as well as increased physical sputtering, despite a lack of increase in RF rectified sheath potential.

Original languageEnglish (US)
Article number126024
JournalNuclear Fusion
Issue number12
StatePublished - Dec 2023


  • ICRH antenna
  • impurity emission
  • ion heat flux
  • particle in cell
  • plasma surface interactions
  • radio-frequency plasma sheath
  • RF sheath impedance

ASJC Scopus subject areas

  • Nuclear and High Energy Physics
  • Condensed Matter Physics


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